This document relates to wireless communications, and more particularly to techniques and systems for implementing a wireless communications link that can provide additional multipath fading protection.
FIG. 1 shows an example of a 3-hop repeater for wireless communications. The 3-hop repeater has a middle hop (hop 2 in FIG. 1) which is “autonomous” from hop 1 that exists between a network unit 102 and Base Transceiver Station (BTS) (i.e. “network transceiver”), and hop 3 that exists between a user unit 104 and a Mobile Station (MS) (i.e. “user transceiver”). That is, the waveform envelope and bandwidth is different in the middle hop compared to the cellular waveform envelope and bandwidth that exist in hops 1 and 3.
The reason for this change of waveform is to modulate the original signal such that the fading in the middle hop (hop 2) can be mitigated by the use of broadband modulation such as Orthogonal Frequency Division Modulation (OFDM), so that the repeated signal is only subject to two fading hops (hops 1 and 3). A third additional hop that imposes yet another fading pattern on the original signal, be it Rayleigh or Rician, can degrade the end signal considerably and push the required fading margins on all three hops sufficiently high to render the repeater ineffective.
OFDM has been the preferred modulation choice for operating in UNII band for systems such as 802.11a and 802.11n and WiMax, as OFDM is very resilient in multipath channels and benefits from inherent frequency diversity. As the operational requirements in the UNII band are only limited to transmit power and spectral emission mask emission, “wideband digital modulation” and a minimum of 1 Mbits/s data rate, there are no regulatory or any other requirements that prohibit the use of other broadband digital modulation schemes such as Spread Spectrum.